Bi-lobular self-thread forming fastener

ABSTRACT

There is disclosed a novel self-thread forming fastener having a threaded shank portion, which includes a holding section having a straight thread of generally circular cross section, and a thread forming section including an underdeveloped thread formation providing a bi-lobular thread cross section symmetrically disposed with respect to the axis of the fastener. The thread forming section includes several thread turns disposed on a taper, with the bi-lobular thread merging with the circular thread formation of the holding section. The thread formation of the holding section has a radial height which is at least as great, or greater than, the maximum radial height of the bilobular thread formation on the forming section. Accordingly upon engagement of the holding section with a female thread formed by the forming section, flank to flank interference will be obtained to provide a locking action. Also disclosed is a novel method for fabricating the above-discussed fastener.

United States Patent [1 1 Carlson A Apr. 22, 1975 1 Bl-LOBULARSELF-THREAD FORMING FASTENER [75] lnventor: Raymond B. Carlson,Rockford. Ill.

[73] Assignee: Textron Inc.. Providence. R1.

[22] Filed: Dec. 29, 1972 [21] Appl. No.: 319,511

[52] US. Cl 85/46; 10/10 R; 151/22 [51] Int. Cl. Fl6b 25/00; Fl6b 33/02[58] Field of Search 85/41. 46. 47. 48; 151/22;

Primary Examiner-Marion Parsons. Jr. Attorney, Agent. or F irm-Olson.Trexler. Wolters. Bushnell & Fosse. Ltd.

[57] ABSTRACT There is disclosed a novel self-thread forming fastenerhaving a threaded shank portion, which includes a holding section havinga straight thread of generally circular cross section. and a threadforming section including an underdeveloped thread formation providing abi-lobular thread cross section symmetrically disposed with respect tothe axis of the fastener. The thread forming section includes severalthread turns disposed on a taper, with the bi-lobular thread mergingwith the circular thread formation of the holding section. The threadformation of the holding section has a radial height which is at leastas great. or greater than. the maximum radial height of the bi-lobularthread formation on the forming section. Accordingly upon engagement ofthe holding section with a female thread formed by the forming section.flank to flank interference will be obtained to provide a lockingaction. Also disclosed is a novel method for fabricating theabove-discussed fastener.

3 Claims, 13 Drawing Figures HOLDING F ORMING PATENTEDAPRZZ I975 saw 3 o'3 FIG.9

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Bl-LOBULAR SELF-THREAD FORMING FASTENER BACKGROUND OF THE INVENTION Thepresent invention relates to self-tapping fasteners, and moreparticularly to an improved, novel fastener and method of manufacturing.

Self-tapping fasteners or screws are widely known and used in the art,and fall into two general categories; namely, thread cutting and threadforming fasteners. The former type forms the female thread in a pilotaperture by a metal removal process, that is metal is actually cut fromthe aperture walls to form the female thread. The latter type offastener, a somewhat more recent development, employs a cold-formingoperation, wherein the female thread is swaged into the wall of thepilot aperture. It is to this latter type of selftapping fastener thatthe present invention is directed.

One form of self-tapping fastener of the cold-working type, employs aforming section having a thread form which includes a plurality oflobes. That is, the thread has a crest formation of an arcuate polygonalshape providing alternate high and low portions; said high portionsdefining the lobes, the low portions the radial relief of the threadform. Accordingly, as the fastener is engaged with the wall of the pilotaperture the high portion or lobes will cold-work the metal with theradial relief providing space permitting the metal to flow duringformation, relative to the thread areas in contact. One commercialdesign of this type fastener utilizes a thread forming cross section ofa generally triangular configuration with the lobes or high points ofthe thread crests corresponding generally to the apecies of a triangle,with the sides or low points of thread form corresponding to the sidesof the triangle. This type of self-tapping fastener is designated atri-lobular, examples of which are illustrated in U.S. Pat. Nos.3,246,556; 3,681,963 and 3,180,126.

In addition to the tri-lobular type of self-tapping fastener, attemptshave been made to provide a commercially acceptable bi-lobular fastener.In this regard, the theory is that by reducing the number of lobularportions, the areas in contact are correspondingly reduced while theamount of radial relief is increased, which factors materially reducethe driving torque. Examples of the aforementioned prior artbi-lobularfastener can be found in U.S. Pat. Nos. 3,192,819 and 3,398,625 both ofwhich will be discussed hereinafter.

While the fasteners of the tri-lobular design are adequate in that theyprovide relatively low driving torques, they are expensive to produceand their holding strength leaves something to be desired. Accordingly,in an attempt to obtain both low driving torque and maximum holding orstripping strength it was proposed to employ a section of straightcircular thread in conjunction with the tri-lobular forming thread, saidstraight thread to be engaged with the female thread of the pilotaperture after formation by the tri-lobular thread form. However simplein theory this concept appears, numerous problems were encountered inthe production of such a fastener by commerical thread rollingprocedures and apparatus. More specifically,

with the method as taught by U.S. Pat. I 20. 3,246,556 'mentioned above,special dies had to be constructed; 'with the fastener of U.S. Pat. Nos.3,180,126 and 3,681,963; oscillation of the die blanks during rollinghad to be accommodated, or more appropriately, tolerated.

The use of special dies is a factor which materially increases the priceof the finished product, as the cost of such dies is extremely high. Asto oscillation of the blank during formation, this factor gives rise toa number of problems or disadvantages. More specifically, the threadportion on opposite sides of the point of oscillation are necessarily oflimited length. Further, where a relatively long fastener iscontemplated, actual oscillation cannot be tollerated. In theseinstances, the fastener is actually cold worked, or bent axially by thedies during rolling, which adversely effects the thread quality. In bothinstances, excessive die wear results which materially shortens the lifeof the die and necessitates costly replacement.

While both U.S. Pat. Nos. 3,192,819 and 3,398,625, teach bi-lobularself-tapping fasteners, these fasteners suffer from one or more of theabove-mentioned disadvantages. The fastener illustrated in U.S. Pat. No.3,192,819 has a bi-lobular thread formation along its entire length. Thethread formation is obtained by employment of a method wherein a roundblank is used in conjunction with specially constructed, scalloped dies.Obviously, die undesirable costs are excessive, an entremely underirablefactor. Further, the fastener as taught by this patent does not employ aholding section. The fastener disclosed in U.S. Pat. No. 3,398,625utilizes a bi-lobular forming section and a rearwardly disposed sectionhaving a thread form of a generally circular cross-section. The fasteneris fabricated using an eliptical blank and standard thread rolling dies.During rolling the dies must be maintained at a slight offset or angleto the axis of the fastener. This is extremely critical and is requiredto provide increased pressure on the eliptical blank rearwardly of thework entering end so that this rear portion can be cold-worked to acircular configuration. As such, the circular thread thus produced inthe rearward portion of the fastener. has a crest diameter less thanthat of the thread forming section. That is to say, the circular threadon the fastener as disclosed will not engage the female thread of thepilot aperture with the sufficient flank interference to produce alocking action or increase holding strength; in fact, this is the statedpurpose as a low running torque is desired.

The present invention contemplates a novel fastener design and method ofmanufacturing which overcomes many of the disadvantages of the priorart, fasteners, and methods, while attain most of the advantages. Morespecifically, the present invention provides a fastener which utilizes asymmetrical bi-lobular forming section capable of forming a femalethread while providing for relatively low driving torque in the process.Further, there is provided a holding section disposed rearwardly of thethread forming section, which employs a straight, circular threadformation capable of being engaged with the female thread formed by thebilobular threaded section to produce maximum holding strength. Inaddition, with the method of the present invention, a blank of specialdesign is employed which permits the roll threading of the fastener withconventional dies. This blank incudes a section of generally circularcross section, in conjunction with a symmetrical bi-lobular section. Assuch, the bi-lobular thread formation obtained upon rolling is alsosymmetrical. Furthermore, the thread rolling operation may beaccomplished without oscillation of the fastener blank; nor is itrequired that the dies be offset axially. As such, not only is die lifeincreased by the elimination of oscillation and offsetting, but nolimitation is placed on the length of the circular and bi-lobularthreaded sections. Accordingly, if desired, the fastener may be employedwith a relatively long lead-in or pilot portion of a circularconfiguration, such as that taught in U.S. Pat. No. 3,633,455.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of a fastenerconstructed in accordance with the present invention;

FIG. 2 isa top view of the fastener of FIG. 1 illustrating the threadformation rotated 90 with respect to FIG. 1;

FIG. 3 is a partial sectional view of the fastener of FIG. 1 beingengaged in a pilot aperture, the view being taken approximately alongthe line 33 of FIG. 1;

FIG. 4 is a partial sectional view of the holding section of thefastener of FIG. 1 engaged in a pilot aperture, said view being takengenerally along the line 4-4 of FIG. 1;

FIG. 5 is a side view of a blank employed in forming the fastener of thepresent invention;

FIG. 6 is a top view of the blank of FIG. 5;

FIG. 7 is an end view of the blank of FIG. 5;

FIG. 8 is a somewhat schematic showing of the blank of FIG. 5, engagedbetween a pair of thread rolling dies, illustrated in phantom;

FIGS. 9-11 are partial sectional views illustrating the rolling of thebi-lobular thread on the blank of FIG. 5; FIG. 12 is a side view of ablank similar to that of FIG. 5, but employing an elongated circular endportion adapted to have a pilot thread formed thereon; and

FIG. 13 is a fastener fabricated in accordance with the presentinvention, from the blank of FIG. 12.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT With reference to FIGS. 1 and2, a fastener constructed in accordance with the present invention, isillustrated. As will be explained, FIG. 2 illustrates a top view of thefastener 20 of FIG. 1, the fastener in effect having been rotated 90relative to said FIG. 1. The fastener 20 includes a driving head 22 anda threaded shank portion 24. The driving head 22, as illustrated, is ofthe hex head type, it being understood that various alternate forms ofdriving heads may be employed.

The threaded shank 24 of fastener 20 includes a holding section,designated generally 26, and a thread forming section 28. Holdingsection 26 is disposed generally adjacent the driving head 22; it beingunderstood that an unthreaded neck portion of substantial length may beemployed intermediate the head 22 and the holding section 26, ifdesired. The thread forming section 28 is disposed proximate the workentering end of the fastener, and forward of the holding section 26.

As mentioned above, in use the fastener forming section 28 will beengaged with an unthreaded pilot aperture formed in a workpiece (notshown in FIGS. 1 and 2) to form a female thread in said aperture wall.Upon advancement of the fastener into the pilot aperture, the holdingsection 26 will come into engagement with the female thread thus formedby the forming section 28. This engagement will produce flank-to-flankengagement of sufficient degree to attain the desired locking action.The manner in which this holding or locking action is attained will beconsidered hereinafter.

Considering first the thread forming section 28, the thread turns onthis section which are designated 30, are of a specific configurationdesigned to facilitate forming of the female thread in the pilotaperture. More specifically, each thread turn 30 is ofa bi-lobularconfiguration. In this regard, the thread turns 30 are what are termedin the art as underfilled. In this regard, each thread turn30 includes apair of lobe portions 32 and a corresponding pair of side portions 34which merge smoothly with said lobes. Looking to FIG. 1, the lobeportions 32 are shown in profile, the threadheight of said lobes 32being indicated at 36. The term thread-height is employed to indicatethe perpendicular distance from the thread root to the thread crest. InFIG. 2, which in effect illustrates fastener 20 rotated with respect toFIG. 1, the side portions 34 of the thread turns 30 are illustrated inprofile, the threadheight of said side portions being indicated at 38.Accordingly, upon comparison of FIGS. 1 and 2, it can be seen that thethread-height of the side portions 34 is considerably less than that ofthe lobe portions 32. This difference is the radial relief of each...;ead turn, as referred to previously.

The thread turns 30 on the forming section 28, as illustrated, include anumber of individual turns, 30 and 30 disposed on a taper, and mergingsmoothly with the thread turns 30 rearwardly thereof, which latter turnsare of a relatively straight configuration. It can be seen that thecrest formtion of the lobes and side portions 32 and 34 of the taperedthread turns 30 and 30" are somewhat less developed than that of thestraight thread turns 30 on the forming section 28.

The tapered thread turns 30 and 30 thus provide for a gradual increasein the amount of metal being swaged or cold worked in the aperture wallduring for mation of the female thread. The straight thread turns of theforming portion 28 in effect size the female thread to the desired,final dimension. If desired, all of the threads on the forming section28 could be disposed on a gradual taper with only the last lobe portion32 being used to size the female thread to its final dimension.

The holding section 26 comprises a plurality of thread turns 40 whichare substantially fully developed and are of a circular cross section,as compared to the bi-lobular thread turns 30 on section 26. The threadturns 40 have a generally constant thread height about their peripheralextent, there being a general absence of any radial relief. In addition,the radial height of the threads 40, that is the distance from the axisof the fastener to the thread crest, is at least as great as the maximumradial height of the lobe portions 32 of thread turns 30 on formingsection 26. Accordingly, the thread turns 40 will have substantialflank-to-flank contact when engaged with the female thread of the pilotaperture thus formed by the bi-lobular thread turns 30.

With a bi-lobublar thread, initial coaxial engagement of the fastener inthe pilot aperture is important. While slight misalignment can betolerated, if this is substantial, the amount of material beingcold-worked during driving is materially increased, producing excessivedriving torques. To assure some measure of the alignment initially,fastener 20 includes a circular nose portion, designated 42. The noseportion 42 has a partial thread formed thereon and in the embodiment ofFIGS. 1 and 2, this thread extends for only a relatively short distance,approximately one-half to two-thirds of a thread turn. The thread formedon the nose portion 42 is circular and merges smoothly with thebi-lobular thread of the tapered thread turns 30 and 30". The radialheight of the thread formation on nose portion 42, at its forwardmostextent is less than the radius of the pilot aperture and increases to aradial height sufficient to achieve initial purchase with the walls ofthe pilot aperture before merging with the bi-lobular thread turns 30 nd30" on the forming section 28. As such, engagement of the pilot portion42 with the aperture wall will provide'a lead-in for the bi-lobularthread turns 30 and will assure the desired initial alignment. As willbe discussed in more detail with regard to FIGS. 12 and 13, a moreextensive nose or lead-in portion may be employed if desired.

In FIGS. 3 and 4 there is illustrated the situation existing uponengagement of the fastener in a pilot aperture. It should be noted, thatthese figures correspond generally to sections taken at lines 33 and 44of FIG. 1, but are of a somewhat schematic nature for purposes ofdiscussion and illustration. Such is the case, in that the thread turnsand 40 are disposed on a spiral or helix, and a perpendicular sectionwould not expose to view the entire thread turn, as is shown in FIGS. 3and 4.

In FIG. 3, the forming section 28 of the fastener 20 is shown as engagedwith a workpiece 46 having a pilot aperture therein. This partialsectional view is taken generally at the line 3-3 of FIG. 1, such that afemale thread 47 has been substantially completely formed in saidworkpiece; the crest of said thread being illustrated in dotted outline.In this regard, the lobes 32 of the bi-lobular thread turns 30 willcold-work or swag the material of the aperture wall to form the femalethread 47. The difference in the thread-height 38 of the side portions34 and the thread-height 36 of the lobes 32 is best illustrated herein,with the difference in said thread heights being the radial relief. Itshould be noted that the female thread thus produced is of a generallycircular configuration as to both the root and crests thereof, this isdue to the symmetrical nature of the bilobular thread turns 30, with theroot diameter of female thread 47 corresponding to approximately twicethe maximum radial height of said bi-lobular thread turns 30.

FIG. 4 is a view generally similar to FIG. 3, but illustrating thesituation obtained upon engagement of the thread turns 40 of the holdingsection 26 with the female thread 47 as formed by thread turns 30. Inthis view, it can be seen that the thread turns 40 are of a generallycircular configuration. Accordingly, since the radial height of thethread turns 40 is at least as great, or greater than the maximum radialheight of the bilobular thread forming turns 20, substantialflank-toflank engagement is obtained about the full extent of theengaged thread turns 40. This engagement will resist inadvertentwithdrawal of the fastener and improve the holding and strippingstrength thereof, over that of the prior art designs.

Referring now to FIGS. 5 and 6, the blank from which the fastener 20 ofFIG. 1 is formed is illustrated and designated generally 50. Tue blankhas the driving head 22 formed thereon, and includes an elongatedunthreaded shank portion 52. The shank portion 52 of the illustratedembodiment is divided into a plurality of distinct sections. Adjacent tothe head 22 is a first section 54 of generally'circular cross sectionwhich section corresponds to the length of the holding section 26dicussed previously. Immediately forward of the circular section 54 is astraight bi-lobular section 56. In this regard, the maximum diameter ofthe section 56 taken through the lobular portions thereof issubstantially equal to that of the circular blank portion 54. On theother hand, the diametrical distance taken through the flat or side areaof this portion is considerably less than the diameter of the circularsection 54. Forward of the straight bi-lobular section 56 is a taperedbi-lobular section 58 which merges with a circular nose section 60. Arelatively short sloping transition section 62 joins the side portionsof the straight bi-lobular section 56 with the circular shank section 54which permits threads to be rolled smoothly and continuously over thelength of the blank despite the differences in crosssectionaldimensions. The second and third sections 56 and 58, will provide thethread forming portions 28 of the fastener and conform generally inshape to the thread turns 30, i.e. bi-lobular in shape. More specifically, a sectional view taken through either of the respective blanksections will include a pair of diametrically opposed arcuate lobesseparated by arcuate, somewhat flattened side portions. In bothinstances, however, the respective lobes and side portions of the blanksections 56 and 58 are symmetrically disposed with respect to thelongitudinal axis of the blank.

A thread is formed on the exterior surface of the blank in accordancewith the standard thread rolling procedures as discussed hereinafter,said operation being performed with standard, conventional thread dies.This thread rolling operation is illustrated somewhat schematically inFIGS. 8-11.

Referring first to FIG. 8, the blank 50 is initially disposed between apair of standard thread rolling dies 64 and 66, illustrated in phantom.The dies 64 and 66 include opposed die faces 68 and 70 respectivelywhich are generally flat and have a plurality of thread forming ridges71, shaped and arranged thereon in the usual manner, as is known in theart. The ridges 71 at the entering end of the dies are quite shallow andincrease in depth in the direction of movement of the blank 50 relativeto the die surface during rolling. Accordingly, a gradual penetration ofthe blank 50 is effected which increases to the full thread depth priorto blank 50 leaving the dies.

The blank 50 is disposed between the dies 64 and 66 in the conventionalmanner, and the dies are moved relative to each other thereby causingblank 50 to roll relative to each said die. Upon set up, the dies 64 and66 are spaced a prescribed distance apart which will insure that theridges 71 penetrate the die blank 50. Accordingly, as the blank rollsrelative to the dies, the thread forming ridges 71 will cold work theblank material to form a desired thread configuration thereon.

At this point, it should be noted that for proper thread rolling it isimportant that the blank roll smoothly relative to the dies. If anyslippage of the blank is encountered, unacceptable thread distortionarise. This problem of slippage is one that has plagued fastenermanufactures in the rolling of lobular thread formations. In thisregard, with blanks of a lobular configuration along their entirelength, there is a tendency for the blank to slip relative to the dies,rather than to roll smoothly. With the aforementioned prior arttrilobular configuration, considerable care must be taken in theformation of the blank side portion in order to provide sufficientcurvature to enable the blank to roll,

rather than slip during forming. Another manner of overcoming thisproblem is to employ extremely high die pressure, so that the forcescreated by engagement of the die with the blank will overcome anytendency of the blank to slip. Obviously, this method is undesirable asit shortens die life. With the present invention, these prior artproblems are, to a great extent, eliminated. That is to say, the diepressures employed may be no greater than that normally employed withstandard, round blanks. Also, the tolerances that need be observedduring formation of the lobularblank sections are not critical toattainment of proper rolling. In addition to these problems, numerousother problems are eliminated by the present invention, as will beapparent from the foregoing discussion.

Any tendency of blank 50 to slip during thread rolling is completelyeliminated by the employment of the circular blank section 54. In thisregard, the thread forming ridges 71 of the die blanks 64 and 66, willcontinuously engage said circular section 64, thus providing sufficientforce to keep the blank 50 rolling smoothly during the entire threadforming operation. This aspect of the invention is illustrated in FIGS.9-11.

With reference to FIGS. 9-11, there is shown somewhat schematically, theconditions existing during various stages of the rolling of blank 50.For reference purposes, a datum line 72 has been provided through theopposed lobe portions of the straight bi-lobular section 56. Further,dotted lines 74 and 76 indicate the maximum depth of the thread ridges71.

Looking first to FIG. 9, it will be noted that datum line 72 is disposedgenerally vertically with the ridges 71 penetrating the lobular portionsof section 56 to'a maximum extent. As to blank 50, rolls, datum line 72will move to a position illustrated in FIG. 10 disposed approximately at45 to the vertical. In this condition, while the thread forming ridges71 are still in full engagement with the straight circular section 54 ofthe blank, the degree of penetration of said ridges in the bilobularsection 56 is reduced. In FIG. 11, the blank 50 is rotated 90 from theposition illustrated in FIG. 9, with the datum line 72 disposedsubstantially horizontally. In this condition, the thread forming ridges71 are no longer engaged with the lobular portions of section 56, butare engaged with the side portions of section 56; as such the degree ofpenetration is at a minimum. It will be noted that throughout the threadrolling operation, the degree of penetration or engagement of the ridges71 with the circular portion 54 of the blank 50 remains constant, whichfactor produces continuous, smooth rolling of the blank 50.

Referring again to FIGS. 1 and 2, it will be recalled the lobularsections 32 of thread turns 30 were completely filled while the sideportions 34 were underfilled. This condition results due to the degreeof penetration of the thread forming ridges 71, as illustrated in FIGS.9-11. That is to say, where the degree of penetration is to the fullextent, such as shown in FIG. 9, the thread form thus produced will besubstantially, completely filled. On the other hand, where the degree ofpenetration is at a minimum, as is the case with the side portions ofthe blank, the condition'illustrated in FIG. 11 the resulting threadformation will be underfilled. It

should also be noted that where the blank portion is undersized, such asthe nose portion penetration by ridges 71 is also insufficient toprovide a completely filled or formed thread.

In addition to the provision for continuous rolling of the blank 50during thread forming, certain additional features are illustrated inFIGS. 9-11. First of all, it should be noted that throughout the rollingoperation, the degree of engagement are the ridges 71 with the opposedlobes of the blank section, is substantially equal for all orientations.Thus, the forces established during cold rolling of the blank materialwill also be of approximately equal magnitude and oppositely directed.Accordingly, the dynamic stress acting upon the dies 64 and 66 are equalat all times, thus providing a dynamically balanced condition.

This dynamic balance is also manifest in another manner, that is theelimination of blank wobbling. In this regard, the longitudinal axis 73of the blank 50 does not change positions with respect to the diesduring rolling. This is a significant feature, in that wobbling of theblank 50 will limit the length of the fastener and also produceexcessive die wear thereby shortening the effective life of the die.Since no wobbling of the blank 50 is encountered, unlike the prior artstructures and methods, there is no limit upon the length of the variousfastener sections that can be formed. Thirdly, the elimination of anytendency of the blank to wobble or slip improves the quality of thethread thus formed.

Attention is now directed to FIGS. 12 and 13 wherein a modified form ofthe present invention is shown. In this regard, attention is invited tothe previous discussion of the problems encountered with the prior artfasteners and methods due to wobbling of the blank during forming. Itwill be recalled that' with the prior art method the relative length ofthe fastener was limited. As such, relatively long pilot portions couldnot be employed economically. No such limitation exists with the presentinvention. Accordingly, where operating conditions so presribe, aself-tapping lobular fastener can be provided with an elongate workentering pilot portion to facilitate pickup of the forming threads andproduce proper alignment during the initial stages of the thread formingoperation.

A fastener constructed in accordance with the present invention andemploying an elongate pilot portion is illustrated in FIG. 13 anddesignated generally 20a. The blank employed in producing the fastener20a as shown in FIG. 12. Basically, the fastener 20a and the method ofmanufacturing is similar to that discussed above, with one exception.That is, in place of the relatively short circular pilot portion 42, thefastener 20a is provided with a relatively long pilot portion 80, havinga thread configuration which will assure proper alignment and engagementof the thread forming turns with the walls of the pilot aperture. Thepilot portion is preferably, but not necessarily, constructed inaccordance with the teachings of the US. Pat. No. 3,633,455.

The fastener 20a is formed in accordance with the present invention in amanner generally similar to fastener 20 of FIGS. 1 and 2. As such,corresponding structural features of the fastener 20a are designed bythe same reference character as employed with regard to fastener 20,followed by a small a. Accordingly, as shown in FIG. 12, said fastener20a includes a driving head 22a, a holding section 26a, provided bycircular thread turns 40a, and forming section 28a, including bi-lobularthread turns 30a. Disposed forwardly of the forming section 28a is thepilot portion 80 to be discussed hereinafter.

The blank for forming the fastener 20a is shown in FIG. 12 anddesignated 50a. Similar to the blank 50 discussed previously, the blankSa includes an elongate circular section 54a, a straight, bi-lobularsection 56a and a tapered bi-lobular section 58a. In place of the noseportion 60 of the blank 50, the blank 50a includes an elongate, circularsection 82. As can be seen in FIG. 12, section 82 is of a reduceddiameter, which is required for formation of the under-developed threadform for nose portion 80, discussed hereinafter.

The elongate pilot portion 80 has an underdeveloped thread configuration84 formed thereon. This thread configuration 84 is of a substantiallycircular crosssection and is step tapered along the length thereof. Theradial height of the thread 84 on the pilot portion 80 increases from abeginning height less than the diameter of the pilot hole to a heightsomewhat greater than said diameter, and then merges smoothly with thethread turns 30a of the thread forming portion. Accordingly, as thepilot portion 80 is engaged with the pilot hole, the thread 84 willachive progressively greater purchase with the aperture wallsestablishing proper alignment and thread pickup prior to engagement ofsaid wall by the thread forming turns 30a. Thus, sufficient alignmentand purchase is attained at relatively low levels of driving torqueprior to initiation of the thread forming operation.

The fasteners and method of manufacture illustrated in the drawings anddescribed above constitute preferred forms of the present invention. itis envisioned and contemplated that those skilled in the art may readilydevise various modifications; however insofar as they are covered by theappended claims, they fall within the spirit and scope of saidinvention.

The invention is claimed as follows:

1. A self-thread forming fastener for forming a female thread in a pilotaperture and effecting a locking action with said female thread, saidfastener comprising: a driving head and a shank having a cold rolledthreaded portion, said threaded portion of the shank.

including a thread-forming section, and a holding section, saidthread-forming section having a plurality of helically disposed threadturns formed thereon, each said thread turn being defined by a generallycircular root, a pair of helically disposed spaced flank surfaces whichare disposed at an acute angle to each other converging in a radiallyoutward direction, and a helically disposed crest surface, the height ofsaid crest surface as measured from the root of said thread varyingabout the helix of each turn such that said flank surfaces and saidcrest surfaces define a pair of diametrically opposed arcuate lobeportions and a corresponding pair of diametrically opposed arcuate sideportions for each thread turn, the height of the crest surface for saidside portions being less than that at said lobe portions, said angle ofconvergence remaining substantially constant about said thread turn, andsaid side portions and lobe portions merging smoothly such that saidflank surfaces and crest surfaces for each thread turn are continuousand uninterrupted, thereby providing each said thread turn with atruncated profile in section throughout the entire thread turn, andfurther providing said forming section with a bi-lobular threadformation capable of forming said female thread in the pilot aperture,and said holding section including a plurality of thread turns having acrest formation of a substantially circular configuration, the radialheight of said thread turns on the holding section being at least asgreat as the maximum radial height of the lobe portions on the formingsection, such that upon engagement of the thread turns on the holdingsection the female thread formed by the forming section flank-to-flankengagement will be obtained thereby establishing a locking action.

2. A fastener as defined in claim 1 wherein a number of the thread turnson said forming section are disposed on a taper, with the radial heightof the respective lobe portions increasing in a direction toward theholding section.

3. A fastener as defined in claim 1 further including a threaded pilotend portion of circular cross-section, the thread formation on saidpilot end being disposed on a taper and having'a minimum crest diameterless than the diameter of said pilot aperture and increasing to amaximum crest diameter suffidient to afford engagement with said pilotaperture, said thread on said pilot end portion merging with thebi-lobula'r. thread formation on the thread forming section of "thefastener.

1. A self-thread forming fastener for forming a female thread in a pilotaperture and effecting a locking action with said female thread, saidfastener comprising: a driving head and a shank having a cold rolledthreaded portion, said threaded portion of the shank including athread-forming section, and a holding section, said thread-formingsection having a plurality of helically disposed thread turns formedthereon, each said thread turn being defined by a generally circularroot, a pair of helically disposed spaced flank surfaces which aredisposed at an acute angle to each other converging in a radiallyoutward direction, and a helically disposed crest surface, the height ofsaid crest surface as measured from the root of said thread varyingabout the helix of each turn such that said flank surfaces and saidcrest surfaces define a pair of diametrically opposed arcuate lobeportions and a corresponding pair of diametrically opposed arcuate sideportions for each thread turn, the height of the crest surface for saidside portions being less than that at said lobe portions, said angle ofconvergence remaining substantially constant about said thread turn, andsaid side portions and lobe portions merging smoothly such that saidflank surfaces and crest surfaces for each thread turn are continuousand uninterrupted, thereby providing each said thread turn with atruncated profile in section throughout the entire thread turn, andfurther providing said forming section with a bi-lobular threadformation capable of forming said female thread in the pilot aperture,and said holding section including a plurality of thread turns having acrest formation of a substantially circular configuration, the radialheight of said thread turns on the holding section being at least asgreat as the maximum radial height of the lobe portions on the formingsection, such that upon engagement of the thread turns on the holdingsection the female thread formed by the forming section flank-to-flankengagement will be obtained thereby establishing a locking action.
 1. Aself-thread forming fastener for forming a female thread in a pilotaperture and effecting a locking action with said female thread, saidfastener comprising: a driving head and a shank having a cold rolledthreaded portion, said threaded portion of the shank including athread-forming section, and a holding section, said thread-formingsection having a plurality of helically disposed thread turns formedthereon, each said thread turn being defined by a generally circularroot, a pair of helically disposed spaced flank surfaces which aredisposed at an acute angle to each other converging in a radiallyoutward direction, and a helically disposed crest surface, the height ofsaid crest surface as measured from the root of said thread varyingabout the helix of each turn such that said flank surfaces and saidcrest surfaces define a pair of diametrically opposed arcuate lobeportions and a corresponding pair of diametrically opposed arcuate sideportions for each thread turn, the height of the crest surface for saidside portions being less than that at said lobe portions, said angle ofconvergence remaining substantially constant about said thread turn, andsaid side portions and lobe portions merging smoothly such that saidflank surfaces and crest surfaces for each thread turn are continuousand uninterrupted, thereby providing each said thread turn with atruncated profile in section throughout the entire thread turn, andfurther providing said forming section with a bi-lobular threadformation capable of forming said female thread in the pilot aperture,and said holding section including a plurality of thread turns having acrest formation of a substantially circular configuration, the radialheight of said thread turns on the holding section being at least asgreat as the maximum radial height of the lobe portions on the formingsection, such that upon engagement of the thread turns on the holdingsection the female thread formed by the forming section flank-to-flankengagement will be obtained thereby establishing a locking action.
 2. Afastener as defined in claim 1 wherein a number of the thread turns onsaid forming section are disposed on a taper, with the radial height ofthe respective lobe portions increasing in a direction toward theholding section.